Review on 2D MXene and graphene electrodes in capacitive deionization

The development of the electrochemical desalination technique for saline water by the capacitive deionization (CDI) process is progressing with ever-increasing interest. In the CDI technology, the saline water flows through the spacer sandwiched between the pair of electrodes, and the ions are captured and stored within electric double layers (EDLs) when a voltage between 0.5–2.0 V is applied. When the voltage is reversed or removed, adsorbed ions are released, and electrodes are regenerated. Carbon materials-based electrodes show effective, but limited electrosorption capacity as the ions are removed based on the EDL mechanism only. There is a demand for new advanced materials that can enhance the electrosorption performance of the CDI. The improvement can be achieved by involving other electrosorption mechanisms such as redox pseudocapacitance and intercalation pseudocapacitance mechanisms. Graphene is a 2D nanosheet material, it has a laminar structure and can be stacked up to form slip shape pores in the electrode and allow easy ion motions than other random porous materials. Graphene can also be prepared into a 3D graphene sponge as an interconnected framework. MXene is a new group of materials made of 2D transition metal nitrides, carbides, and carbonitrides. It has exhibited a significant pseudocapacitance behavior, outpacing the energy storage capacity of many other capacitive materials, particularly when normalizing to the electrode's volume. This review discusses (1) the recent advances in graphene materials and their limitation for enhanced desalination performance and (2) the state-of-the-art art developments of MXene as an emerging material for CDI.